The present invention relates, in general, to producing plasmas, and more particularly, to a novel method for producing a plasma containing both beryllium ions and beryllium fluoride ions.
In the past, implanting beryllium (Be) into semiconductor materials had been difficult to achieve. Two primary methods were used to produce beryllium ions which could then be implanted into semiconductor materials. In general, the two techniques utilized an ion implanter's ionization chamber to produce beryllium ions, and then used the implanter's accelerator to emit the beryllium ions at a semiconductor material target thereby implanting them into the target. Equipment preparation for both methods was very time consuming, and could take four to six hours.
One of the methods produced beryllium ions from beryllium crystals. The crystals were used in the ionization chamber as its' cathode while an inert gas, typically argon, was used in the ionization chamber as an ionization agent. The gas was ionized causing it to bombard the beryllium cathode and create beryllium ions. The beryllium ions were then implanted into the semiconductor material. Generation of beryllium ions was very slow as indicated by the low value of beam current, typically 10 nanoamps. per square centimeter, produced during the procedure. With such a low beam current, it took several hours to implant adequate concentrations of beryllium ions into the semiconductor material. In addition to being low, the beam current was also very unstable and fluctuated during implanting operations. As a result, the implanting equipment had to be constantly monitored and adjusted to provide consistent implanting of the beryllium ions. Therefore, this method of generating beryllium ions resulted in time consuming equipment preparation, low beam currents, and unstable beam currents.
Another method of obtaining beryllium ions utilized a standard tantalum cathode instead of a beryllium cathode to create the plasma. Beryllium chloride (BeCl.sub.2) crystals were heated to produce beryllium chloride gas, then an ionization agent, typically argon, was ionized to activate the beryllium chloride gas thereby producing beryllium ions which were then implanted into the semiconductor material. Since beryllium chloride was more volatile than beryllium, it was easier to ionize and produced a higher and a stabler beam current than was produced using beryllium crystals as a source. But, beryllium chloride easily hydrolyzed to form hydrochloric acid which was very corrosive and damaged the ion implanting equipment. Therefore, the use of beryllium chloride as a source of beryllium ions not only resulted in time consuming equipment preparation, it also produced corrosive acids which damaged the implanting equipment.
Once beryllium was implanted as a P-type dopant into a semiconductor material, it had a high migration rate within the material. As various semiconductor processing steps exposed the implanted material to elevated temperatures, the beryllium easily migrated out of the areas into which it was implanted. Consequently, a material such as fluorine was used to lockup the beryllium and increase the energy required to activate or create migration of the implanted beryllium. A separate implant operation subsequent to the beryllium implant was used to implant the fluorine. Using a separate fluorine implant operation increased manufacturing costs of semiconductor devices that used beryllium as a dopant.
Accordingly it is desirable to have a method for implanting beryllium that is not corrosive, that has an easily controlled beam current, that has minimal equipment preparation procedures, and that permits simultaneous implanting of both beryllium and fluorine.